Technical Field
The present invention relates to a method for forming a multilayer coated film where in coating an automotive body, the multilayer coated film is excellent in the chipping resistance and the adhesive strength when bonding to a member.
Background Art
On an automotive body, particularly, on an exterior panel part thereof, a multilayer coated film including an undercoat film excellent in the anticorrosive property, an intermediate coated film excellent in the smoothness and chipping resistance, and a top coated film excellent in the appearance and resistance to an environmental load is generally formed for the purpose of imparting excellent anticorrosive property and appearance.
In an automotive manufacturing specification, an adhesive layer is formed on the top coated film, and a glass member such as front glass or rear glass is generally fixed thereto with an adhesive.
One of the performances required of the multilayer coated film of an automobile is chipping resistance (chipping: damage to a coated film caused when hit by a small pebble, etc. flying from road), and a function is imparted mainly to an intermediate coated film with an attempt to enhance the chipping resistance, but in recent years, higher-level chipping resistance is demanded to satisfy needs of the market.
However, when the intermediate coated film is, for example, softened with the intention to raise the level of chipping resistance, the cohesive force of the intermediate coated film is reduced and in turn, the multilayer coated film in the bonded part to a member such as glass member may cause cohesion failure, or separation may occur between an electrodeposition coated film and the intermediate coated film, disadvantageously leading to defective bonding to a member.
For example, Patent Document 1 discloses a method for forming a coated film using an intermediate coating composition containing (a) a urethane-modified polyester resin obtained by reacting an aliphatic diisocyanate compound with a hydroxyl group-containing polyester resin obtained by polycondensation of an acid component containing 80 mol % or more of isophthalic acid, and a polyhydric alcohol; (b) a melamine resin; (c) a hexamethylene diisocyanate compound blocked by an active methylene group-containing compound; (d) a nonaqueous dispersion resin having a core-shell structure; and (e) a flat pigment.
Patent Document 2 discloses a multilayer coated film on an external panel of an automotive body, etc., which is a multilayer coated film having formed therein, in the following order, an electrodeposition coated film, an intermediate coated film, a base coated film and a clear coated film, wherein at −20° C., the intermediate coated film has a Young's modulus of 35,000 kg/cm2 or more and an elongation at break of 2% or less and the clear coated film has a Young's modulus of 35,000 kg/cm2 or less and an elongation at break of 5% or more.
However, in the method for forming a coated film of Patent Document 1 and the multilayer coated film of Patent Document 2, the chipping resistance may be insufficient.
Patent Document 3 discloses a multicoat coating system including an undercoat film composed of an undercoating composition containing a specific melamine resin and a polyester binder, and a multilayer coating including an adhesive layer composed of an adhesive of specific isocyanate base.
Patent Document 1: JP-A-2003-211085
Patent Document 2: JP-A-2006-239535
Patent Document 3: JP-T-2012-524673 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”)